JP Patent Publication (Kokai) Nos. 2000-140638, 1997-248458, and 2000-189799 disclose a catalyst, which purifies exhaust gas from the internal combustion engine of an automobile or the like.
JP Patent Publication (Kokai) Nos. 2000-140638 and 1997-248458 disclose that when a catalyst containing elements acting as NOx-storaging materials, such as alkaline-earth metals or alkaline metals, is used in exhaust gas at a high temperature, a solid phase reaction occurs between porous carriers such as alumina and the element acting as an NOx-storaging material, so that the specific surface area of the carriers and the NOx-storaging ability are decreased. Further, it is also disclosed that to prevent a solid phase reaction from occurring between a carrier and an element acting as a NOx-storaging material, a composite oxide as represented by MgO·nAl2O3, is used as a carrier. JP Patent Publication (Kokai) No. 2000-189799 discloses that a composite oxide carrier having the Spinel structure as represented by M·Al2O3 (where M is an alkaline-earth metal) is used, so as to suppress the sintering of platinum composite oxides and thus to suppress the particle growth of platinum clusters.
A catalyst comprising alkaline metals and/or alkaline-earth metals such as sodium, potassium, lithium, cesium, strontium and barium has been expected to be useful as an exhaust gas purification catalyst for an internal combustion engine, particularly for an internal combustion engine that is operated in a lean atmosphere. If the catalyst's performance to purify exhaust gas can be enhanced, its usefulness is further increased.
We have studied catalysts containing alkaline metals or alkaline-earth metals, such as sodium, potassium, lithium, cesium, strontium and barium. Over the course of this study, we have confirmed that these alkaline metals or alkaline-earth metals tend to sinter at a high temperature, particularly at 800° C. or more. We have also revealed that a catalyst, wherein alkaline metals or alkaline-earth metals sinter, is no longer able to maintain its high performance of purifying nitrogen oxides (NOx).
An object of the present invention is to suppress sintering of the alkaline metals and/or the alkaline-earth metals in a catalyst comprising alkaline metals and/or alkaline-earth metals such as sodium, potassium, lithium, cesium, strontium and barium, with respect to not only the catalyst comprising alkaline metals and/or alkaline-earth metals, but also a catalyst comprising a component that causes sintering at a high temperature.
Another object of the present invention is to suppress the sintering.